The principle of operation of such a transmission is well known. Thus, the V grooves of the relay pulley are of variable size because of the displacement of the movable member axially on the hub of the relay pulley. Each relative displacement of the drive, driven and relay pulleys relative to each other gives rise to an external or internal radial displacement of the belt of one of the transmissions in one of the grooves of the relay pulley and substantially simultaneously of a directly proportional radial displacement in a reverse direction of the belt of the other transmission in the other groove under the action of the displacement of said movable member. This variation of the diameter of engagement of the belts in the grooves of the relay pulley generates the desired speed change.
The principal drawbacks encountered in this type of transmission are high wear of the belts and of the assembly of members constituting the transmission, as well as large power loss by friction such that the overall output of the transmission is mediocre. To permit a reduction in wear of the constituent members of this transmission and correspondingly to increase the output of the transmission, it is necessary that the belts of the two transmissions be slightly stretched when the engine stops and that their tension increases proportionately to the increase of the couple transmitted. Because of this problem, a certain number of manufacturers have designed tensioning devices for the belt at the moment of starting the transmission. However, none of these devices is satisfactory to the extent that all these devices include fragile parts, in particular springs, as again shown in U.S. Pat. No. 4,176,560, or tensioning rollers.